The prevalence of breast cancers compared to other malignancies suggests that the breast epithelium is uniquely susceptible to development of tumors. Our laboratory has found that the p53 tumor suppressor protein is sequestered in the cytoplasm of quiescent mammary epithelium and responses to gamma-irradiation are compromised. However, the activity of p53 was also shown to vary with hormonal treatments and developmental state. Therefore, we propose that susceptibility of the breast epithelium to tumorigenesis is due, in part, to compromised function of wild type p53 in quiescent ductal epithelium; and that the latent p53 can be activated by endocrine factors. Three specific aims are proposed to develop methods for analysis of the mechanisms responsible for cytoplasmic sequestration and activation of p53 both in vivo and in vitro. Specific Aim 1: Mice will be injected with hormones in an effort to identify the minimal treatment required to activate latent p53. Phosphorylation and acetylation patterns of p53 will be analyzed. Specific Aim 2: Culture methods will be developed for mammary epithelia (mouse and human) to dissect the pathways responsible for cytoplasmic sequestration of p53 and activation by hormones. The effects of extracellular matrix, stromal cells and hormonal treatments will be tested using primary cultures of mammary epithelium. These experiments will determine whether endocrine treatments act directly on the epithelium or by indirect mechanisms. The minimum region of p53 required for cytoplasmic localization will be identified using a deletion series of GFP-p53 constructs expressed in the primary cultures. Specific Aim 3: The p53(null) mammary transplantation model will be used to determine whether chemopreventive hormonal treatments are effective in reducing the incidence of tumors arising from p53(null) mammary transplants. This will determine whether the agents act via p53-dependent or -independent pathways.